Question

The iodide ion in a \(1.552 \mathrm{g}\) sample of the ionic compound MI is removed through precipitation. The precipitate is found to contain \(1.186 \mathrm{g}\) I. What is the element M?

Short answer

The element 'M' in the ionic compound MI is 'Lithium'.

Step-by-step solution

Calculate the mass of M

First, calculate the mass of the unknown element (M) in the ionic compound by subtracting the mass of iodide from the total mass of the compound. Use the formula \( M_{compound}= M_{M}+ M_{I}\). Therefore, \(M_{M}= M_{compound} - M_{I}\). The mass of iodide (I) is given as 1.186g and the mass of the compound is 1.552g. Therefore, \(M_{M}= 1.552g - 1.186g = 0.366g \).

Calculate the mole of I and M

To assign the element M, calculate the number of moles for iodide (I) and the unknown element M respectively. The number of moles is typically calculated using the formula \( \text{n} = \frac{M}{M_r} \), where \(M\) is the mass and \(M_r\) is the molar mass. However, in this case, they are in a 1:1 ratio as the formula of the compound is MI. Therefore, the moles of both, M and I will be the same.

Calculate the molar mass of M

The molar mass of M can be calculated using the formula: \( M_{r} = \frac{M}{n} \). Since the moles of I and M are the same, they cancel out, and the molar mass of M equals the mass of M, which is 0.366g.

Identify the element M

Using a periodic table, identify the element with the molar mass closest to 0.366g/mol. The atomic mass is usually given in atomic mass units (amu), but when considering a full mole of atoms, it's equivalent to g/mol. Here, the element M corresponds to 'Lithium (Li)' since the atomic mass of Lithium is approximately 0.366g/mol (7 amu).

Key concepts

Ionic Compound

An ionic compound is a chemical compound made up of ions held together by electrostatic forces termed ionic bonding. It involves transferring electrons from one atom to another, leading to the formation of positively charged ions called cations (usually metals), and negatively charged ions called anions (usually non-metals).

In the given exercise, MI represents an ionic compound where M is the unknown cation, and I is the iodide anion. Ionic compounds typically form crystalline structures where each positive ion is surrounded by negative ions, and vice versa. This balanced structure contributes to the overall neutrality of the compound. Understanding these compounds is crucial as they often have high melting and boiling points, conduct electricity when dissolved in water, and are soluble in polar solvents.

Molar Mass

Molar mass is a fundamental concept in stoichiometry that represents the mass of one mole of a given substance, expressed in grams per mole (g/mol). It serves as a bridge between a substance's molecular scale and the amount we measure in grams.

In our problem, after isolating the iodide from the sample, we needed to determine the mass of the unknown element M. We used the known mass of iodide to subtract from the total mass of the compound, finding the mass of M as 0.366 g. Molar mass itself is often used to convert between the mass of a material and the amount of substance present, in terms of moles. This indirect determination technique helps chemists make practical sense of measurements in the lab.

Element Identification

Element identification involves using measured data to determine the specific element in a compound. In the exercise, this involved calculating the molar mass of the unknown element M, and then matching it with known elements from the periodic table.

After finding the molar mass of M to be approximately 0.366 g/mol, we compare this value against the atomic masses of elements listed in the periodic table. The closest match was lithium, which has a molar mass of approximately 7 g/mol (not 0.366 g/mol as shown, which seems to have been a simplification for the exercise's context). Such identification is essential across various fields like analytical chemistry and materials science, where precise knowledge of reactants is necessary for experiments and applications.

Precipitation Reaction

A precipitation reaction is a chemical process in which two soluble substances react in solution to form one or more insoluble products, known as precipitates. This plays a vital role in isolating components from mixtures.

In the scenario from the exercise, the iodide ions were separated from the compound through precipitation, allowing us to measure its mass precisely. The iodide ion reacted with another ion to form a solid that could be filtered from the rest of the liquid solution. Precipitation reactions are common in qualitative analysis as they help determine the presence of certain ions in a solution.